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Κυριακή, 1 Δεκεμβρίου 2013

It Might Be Easier to Find Dying Alien Civilizations

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Dying aliens could broadcasta warning.

Astrobiology is the search for and study of life elsewhere in the universe, and it is obsessed with beginnings. Most experts in the field spend their time concocting theories about the origins of life, because knowing what life requires to get started lets us narrow the list of places we need search for it. However, determining life’s starting conditions is just one aspect of the search for life, and as we see on Earth, life can profoundly affect a planet once it gets a foothold. Rather than ask what life’s beginnings might look like on another planet, astronomers are beginning to wonder how a planet might look as its biosphere comes slowly to its end.

Now, a civilization nearing its apocalypse might broadcast that fate in any number of ways — up to and including literally broadcasting the story in an alien language, warning of technological pitfalls or pleading for help from any species advanced enough to listen. It’s impossible to predict how such a meeting might go, however, so astrobiology focuses less on failed races than on unlucky ones. A naturally dying planet or star will end a species’ run at greatness as surely as any climate crisis or nuclear war, and astronomers might be able to use such an event to find our unluckiest galactic neighbors.

The idea came about thanks to some work with mathematical models of Earth. It was meant to study the evolution of atmospheric phenomena, but by turning the clock forward several billion years, they were able to see an approximation of the Earth’s state at the end its lifetime — and thus, of our lifetime on it. An extinction event can be any number of things, but there are some characteristics that astronomers believe they might share.

For instance, a planet undergoing mass extinctions should be flooded with compounds released by decaying plant and animal matter. Methanethiol, in particular, is thought to only arise from biological sources, and its presence on a planet would instantly bring scientists to high alert. It would be a major indication that we aren’t alone in the universe — or at least that we weren’talone. By the time the light brought us word of the methanthiol flood, the life which gave rise to it may well have died off entirely.

Methane is another signifying gas, and it’s thought that a methane-rich atmosphere could indicate the presence of substantial amounts of microbial life. Actually, running a number of climate models forward by several thousand years, the researchers were able to come up with an end-of-life profile for Earth. They predicted what Earth might look like to far-flung surveyors in the future, looking for ways that life on the modern Earth could be detectible at that time — in a few billion years, microbes may have built up a methane concentration ten times the one we currently breathe.

Thus, in the search for life we may soon be looking for both live planets and dead ones — both those that hold life today, and those which held it at some point. Finding a dead planet is likely a matter of finding planets with atmospheres that differ greatly from expectations, where some unknown force must have intervened. We assume that the unknown force is a working biosphere — something that gives Earth, for instance, an unexpected amount of carbon dioxide. To go into much more specific predictions would be to make assumptions about the natures of any alien species we might stumble across.

Of course, planets can go dead in any number of ways. It seems that Mars was hit by an asteroid and lost its atmosphere to deep space, ending all life on the surface (if indeed any ever existed). On the other hand, planets like Venus have been proposed to have hosted life at some point, too. It will be important to understand not just how life can form, but how the formation of life impacts a planet. Because while finding life’s candidate planets is one thing, finding life itself has been the real goal all along.